1. Field of the Invention
The present invention relates to a developer, a process cartridge and an electrophotographic apparatus such as a copier, a printer, or a facsimile apparatus that uses the developer and process cartridge.
2. Discussion of the Background
It is common practice in an electrophotographic apparatus to form a latent image electrostatically on an image carrier, e.g., photoconductive element, and then to develop the latent image with a developer, i.e., toner with an additive, to produce a corresponding toner image. The apparatus then transfers the toner image to a paper or other recording medium in a transferring operation. The paper then has the toner image fixed thereto by heat and pressure and then is driven out of the apparatus as a recorded sheet.
An electrophotographic apparatus of the type described above is practicable with one of two different charging methods, i.e., a corona charging method (non-contact charging method), and a contact charging method. The contact charging method is advantageous over the corona charger in that it produces a minimum of ozone during operation, and in that it is operable with a low voltage.
The contact charging method holds a charger in contact with the photoconductive element. However, the contact charging method is susceptible to providing poor performance (e.g. non-uniform charging) when the charger becomes dirty with residual developer. With the contact charging method, therefore, the residual toner on the photoconductive element is removed by a cleaning blade so as to prevent the charger from getting dirty. The architecture of conventional contact charging apparatuses is such that the reliability of the charger has an inherent lifetime, after which, the charger should be replaced as part of routine maintenance.
However, the present inventor has determined that the reliability of the contact charging apparatus is also effected by a ratio of additive to toner. This observation was made in light of an identification of a failure mechanism of the conventional apparatuses, evident by observed white spots on a recorded sheet. Moreover, the present inventor observed that with conventional devices, after the removing procedure is performed, a small amount of the residual toner, and a related amount of the additive passes between the cleaning blade and the photoconductive element. This additive becomes particularly troublesome if the mixing ratio of the additive to toner is large because the diameter of the additive is very small relative to the diameter of the main body of the toner. Common additives, such as silica, have a particle size of 0.01 .mu.m to 0.05 .mu.m, while the average diameter of the main body of the toner varies between 5 .mu.m to 20 .mu.m. The present inventor determined that conventional devices do not account for relatively large ratios of additive to toner as being a failure mechanism (limiting the lifespan) of the charging apparatus. Thus, it is the additive, which is added to promote the flow of toner in the developing process, attaches and accumulates on the charge brush (or other charger) and gives rise to non-uniform charging. As a consequence, the charge brush does not adequately charge the photoconductive element and thus an electric potential surface of the photoconductive element, falls short of the normal range (-750 volt) to the abnormal range (-1200 volts, for example). As a consequence, because the photoconductive element is not adequately charged, when the photoconductive element is illuminated with a laser device or other light source so as to form a latent image thereon, the latent image does not produce the desired low voltage i.e., less negative), such that the toner fails to attach to the photoconductive element and consequently, the ultimate image produced by the image recording device, includes white spots thereon.
According to the present inventor's observations of conventional apparatuses, an insufficient amount of additive will prevent the toner from flowing properly, and therefore inhibit the developing process, while too much additive, will limit the lifespan of the charging apparatus, owing to the smaller size additive not being adequately cleaned from the photoconductive element, and contaminating the charging device, therefore preventing the charging device from providing a uniform charge on a photoconductive element. Rather than concluding that the charging element has a limited lifetime, the present observation is that the lifetime is effected by the relative amount of additive to toner.